A comprehensive suite of characterisation services
We have a wide range of optical fibre calibration services, many with UKAS accreditation. We can measure samples of your own fibre or supply calibrated fibre (or reference fibre artefacts) to meet your specific requirements.
NPL can calibrate the chromatic dispersion in single-mode optical fibres from 2 km to 50 km in length, wavelength range from 1260 nm to 1640 nm. This is a UKAS accredited service.
The time taken for radiation to propagate down a single-mode fibre depends on the wavelength of the radiation. Therefore, a pulse, which is made up of a range of wavelengths, broadens as it propagates down the fibre, which limits the minimum usable pulse length and spacing. The variation of propagation time with wavelength is known as 'dispersion', or the 'dispersion coefficient', and it sets an intrinsic limit on the information carrying capacity of the fibre. As the lengths and bit-rates of fibre-optic links increase, accurate knowledge of dispersion properties become more important in determining system performance.
We measure chromatic dispersion according to the IEC reference test method based on the phase shift technique: Chromatic dispersion IEC 60793-1-42 Phase shift technique – Method A.
- Dispersion ±1.5%,
- Zero dispersion wavelength ±0.01 nm
- Dispersion slope at zero dispersion wavelength ±1.5%
Mode field diameter
NPL can calibrate the mode field diameter (MFD) of single-mode optical fibres, both shifted and unshifted fibres. The standard calibration is at 1310 nm and 1550 nm (1618 nm available on request). This is a UKAS accredited service.
The mode field diameter is a measure of the radial intensity distribution of radiation propagating within a fibre. This is a key parameter for predicting properties such as splice loss, microbending loss, cut-off wavelength and waveguide dispersion.
NPL can provide fibre standards and also measure customer supplied samples across a range of single-mode fibre types. Approximately 12 m of primary coated fibre is required and we can accommodate bare fibre as well as fibre mounted in SRM-style holders. We measure mode field diameter according to the IEC reference test method, based on the far field scan technique: Mode field diameter IEC 60793-1-45 Direct Far Field Scan – Method A.
- Best measurement capability: ±0.60 %
- Non-circularity: ±0.05%
NPL can calibrate the effective area (Aeff) of single-mode optical fibres, both shifted and unshifted. The standard calibration is at 1310 nm and 1550 nm (with 1618 nm available on request). This is a UKAS accredited service.
NPL can provide fibre standards and also measure customer supplied samples across a range of single-mode fibre types. Approximately 10 m of primary coated fibre is required and we can accommodate bare fibre as well as fibre mounted in SRM-style holders.
We calculate the effective area of single-mode fibres using the Hankel transform, after acquisition of the far-field scan data using the mode field diameter measurement facility.
- Best measurement capability: ±2.0 %
NPL calibrates the attenuation of single-mode fibre from 1200 nm to 1650 nm in 5 nm steps, and multimode fibre from 800 nm to 900 nm and 1250 nm to 1350 nm in 5 nm steps. This is a UKAS accredited service.
Spectral attenuation is the loss of optical power as it is guided through the fibre. Accurate measurement is essential to maximise the distance between repeater stations. NPL can supply or measure the customer's own fibre, both single and multimode. The NPL-designed Fibre Attenuation Standard is a proven reference standard of high accuracy and reproducibility and can be calibrated for spectral attenuation coefficient uniformity and optical length, with a proven attenuation stability better than ± 0.03 dB.
We measure spectral attenuation according to the IEC reference test method based on the cut-back technique: Spectral Attenuation IEC 60793-1-40. We launch spectrally selected radiation from a grating monochromator into the fibre under test and the transmitted power is recorded as a function of wavelength. The fibre is then cut-back to a few meters without disturbing the launch conditions. The transmitted power through this short length of fibre is again recorded as a function of wavelength, to provide a reference level from which to calculate the spectral attenuation under test.
- Single-mode fibre (SMF) best measurement capability: ± 0.021 dB
- Multi-mode fibre (MMF) best measurement capability: ± 0.02 dB
NPL calibrates the optical length (time of flight) of both single-mode and multimode optical fibres. Standard measurements are made at 1310 nm and 1550 nm for single-mode fibre (up to ~100km), and at 850 nm and 1300 nm for multimode fibre (up to ~4.4 km). Wavelengths between 1260–1640 nm are available on request. This is a UKAS accredited service.
NPL can measure customer supplied fibres or supply reference fibre standards. Bare fibre can be measured, however fibre with FC/PC or FC/APC connectors are preferred. We use a time of flight split pulse technique where the time for an optical pulse to propagate through the fibre under test is measured. The optical length is then calculated using the speed of light and an assumed group refractive index of 1.46000.
Single-mode fibre (SMF) best measurement capability:
- 0.1 km -15 km: (1270 - 1640 nm): ± 0.040 + 1.7x10-5 L
- 15 km - 105km: ± 0.10 + 1.7x10-5 L 1310 (1550 and 1625 nm wavelengths only)
Multi-mode fibre (MMF) best measurement capability:
- 0.1 km – 6.6 km ± 0.3 + 1.4x10-5 L
Attenuation coefficient uniformity
NPL calibrates the attenuation coefficient uniformity of single-mode optical fibres at 1311 nm and 1555 nm up to 14 km. This is a UKAS accredited service.
We can measure customer supplied fibres or supply reference fibre standards. NPL typically supplies ~12 km lengths of spooled fibre calibrated for attenuation uniformity and optical length. Lengths greater than 14 km can also be measured but are not UKAS accredited.
The attenuation uniformity of single-mode fibres can be measured using a calibrated and well characterised optical time domain reflectometer (OTDR) which has been adapted to measure over all polarisation states, therefore removing anomalous features arising from polarisation effects. Measurements are typically made of sequential 1 km sections within the fibre, starting between 0.5 km and 1 km from the end. A value for the measurement uncertainty is obtained from the superimposed graphs, of both single direction and bi-directional data.
- Best measurement capability: 4 km – 14 km ± 0.0060 dB/km.
NPL calibrates the cut-off wavelength of an approximately 2.2 m length of single-mode fibre and 22 m for cable cut-off wavelength measurements for primary coated fibre. This is a UKAS accredited service.
The cut-off wavelength is the shortest wavelength at which only the fundamental mode propagates through the fibre. Measurement of this parameter is necessary to avoid degradation of modal noise of the optical signals carried by a single-mode fibre.
Our fibre cut-off wavelength measurement service uses the internationally-recognised transmitted power reference test method. Spectrally selected radiation is launched from a grating monochromator into a few metres of the fibre under test and the power transmitted recorded as a function of wavelength. The international reference test method defines the cut-off wavelength at which the ratio drops below 0.1 dB. Our cut-off wavelength measurements follow the IEC reference test method: Cut-off wavelength IEC 60793-1-44 – Bend Reference Technique – Annex C.
Our cable cut-off wavelength measurement service uses the transmitted power technique, with measurements performed on 22 m of primary coated, un-cabled fibre which is constrained in loops of radius greater than 140 mm. We follow the IEC reference test method: Cut-off wavelength IEC 60793-1-44 – Transmitted Power Technique Reference Technique – Method A.
- Best measurement capability: ± 2.0 nm
- Cable cut-off wavelength capability: ± 2.0 nm